The electrodialysis of salt solutions aimed at obtaining the relevant acid and base can be carried out in three compartment electrochemical cells, with a central compartment separated form the anodic compartment and the cathodic compartment by means of two ion-exchange membranes, respectively an anion-exchange membrane and a cation-exchange membrane. The ionic species coming from the dissociation of the salt fed to the central compartment migrate across the respective membrane under the effect of the electric field, bringing about the generation of the relevant acid at the anodic compartment and of the respective base at the cathodic compartment. The anodic compartment is also provided with an anode, on whose surface the evolution of oxygen takes place, while the cathodic compartment is provided with a cathode on which the evolution of hydrogen takes place. One of the possible applications of this technology is for instance the electrodialysis of sodium sulphate solutions with production of sulphuric acid and caustic soda, as an alternative of the most common production of caustic soda by sodium chloride brine electrolysis. This process can be applied for example at caustic-consuming sites having no use for chlorine—which in this case would constitute a by-product difficult to handle and store—or whenever a mutually independent production of chlorine and caustic soda is desirable. The electrical consumption associated with the process is nevertheless very high, due to the voltage associated with the overall net reaction—corresponding to water electrolysis with production of hydrogen and oxygen—and to the high ohmic drop in the various components, with particular reference to the anion-exchange membrane. The problem of the excessive electrical consumption was mitigated in the past by replacing the oxygen-evolving anode with a hydrogen-fed, anodically polarised gas-diffusion electrode: in this way, the overall net reaction has a much lower reversible voltage, corresponding to the potential difference between hydrogen cathodic evolution in alkaline environment and hydrogen anodic consumption in a substantially acidic environment. Also this kind of technology failed to meet however the expected success, on one hand because the various overvoltage components adding up in the process lead in any case to a conspicuous energy consumption, on the other hand because of the difficulty in controlling the process, which is characterised by operative voltage fluctuations even at reduced current density (below 2 kA/m2) associated with the difficulty in humidifying the hydrogen flow fed to the anodic compartment in a regular fashion. An irregular hydrogen humidification may lead to a drying out of the relevant membrane—with consequent rise, sometimes steep, of the operative voltage on account of an ohmic effect—or to a partial flooding of the gas-diffusion electrode, with consequent rise of the operative voltage due to inadequate mass transport of supplied hydrogen. In the most serious cases of flooding, the electric voltage applied at the cell poles may lead to the impossibility of supporting the hydrogen consumption anodic reaction and to the sudden onset of oxygen evolution, with destructive effects for the gas-diffusion electrode which is not specifically designed to resist the oxidising action of nascent oxygen. This situation can moreover lead to the formation of explosive mixtures in the anodic compartment, as it will be evident to a person skilled in the art.
Thus there is a need of providing a new device for the electrodialysis of salt solutions of higher energy efficiency and at the same time easier and safer to operate.